In radio broadcast technology, high-frequency amplifiers according to the Doherty principle have been increasingly used in recent years, especially in the case of modulation methods with a non-constant envelope and high crest factor, such as DVB signals. By comparison with conventional amplifiers, they are characterised by a significantly improved efficiency with hardly changed circuit complexity.
In this context, a main transistor, which is generally operated in AB mode, works with small input signals on an increased load resistance in such a manner that it reaches saturation even from a relatively low level, for example, 6 dB below the 1 dB compression point, and therefore operates with maximum efficiency. Above the level threshold specified by the saturation level in the main transistor, a second auxiliary transistor operates in C mode. Through its output signal, it reduces the load resistance of the main transistor. With full level control, the load resistance of the main transistor is accordingly reduced by the ratio between the level threshold and the 1 dB compression point, and the main transistor emits the correspondingly higher power. In the 6 dB example, half the resistance and therefore double the power is obtained.
From the level threshold, the main transistor therefore emits a rising output power in spite of saturation and, in this context, always operates with maximum efficiency. This is reduced only during the operating phases of the auxiliary transistor by its power consumption, but remains significantly higher by comparison with a conventional AB amplifier. With full level control of the amplifier in the signal peaks, both transistors each deliver half of the output power of the system.
The dynamic reduction of the load resistance of the main transistor takes place as follows: both transistors work on the same load resistance, which corresponds to half the system wave resistance, conventionally 25 ohms. In this context, the auxiliary transistor and the main transistor are connected directly to the load via an impedance inverter. At low levels, the auxiliary transistor does not operate. Its output is high ohmic and therefore provides no disturbance. The transistor capacitance is tuned through a matching network and a line. The main transistor works on the load enlarged by the impedance inverter. In the example with a 6 dB threshold, this is therefore 100 ohms. For this purpose, the impedance inverter has a wave resistance of 50 ohms. From the level threshold, the current of the auxiliary transistor superposes the current of the main transistor on the load resistance. Ideally, this occurs from an open circuit, so that it begins to deliver an increasing portion of the output power.
A line dimensioned to one quarter of the operating wavelength is conventionally used as the impedance inverter. This is compensated again in the branch of the main transistor, for example, also by a λ/4-line arranged behind the power splitter or by a 90° power splitter.
To ensure that the impedances from main transistor and auxiliary transistor behind the output matching network are real and high ohmic at the operating frequency, two offset lines are conventionally provided. In this manner, the output matching network can be freely dimensioned. Conversely, the offset line in the case of the main transistor also ensures that the dynamic change in resistance at the input of the impedance inverter, viewed from the main transistor, is transformed, in the 6 dB example, 100 to 50 ohms at the operating frequency in real terms to the drain.
Accordingly, for example, document WO 2012/150126 A1 shows a conventional Doherty amplifier. Although a Doherty amplifier already achieves a better efficiency than a conventional broadband amplifier, its efficiency is also not optimal.